896
UNIT 4
Maintenance of the Body
23
temporary protection against antigens to which the mother has
been sensitized.
Lipid Absorption
Just as bile salts accelerate lipid digestion, they are also essential
for the absorption of its end products. As the water-insoluble
products of fat digestion—the monoglycerides and free fatty
acids—are liberated by lipase activity, they quickly become as-
sociated with bile salts and
lecithin
(a phospholipid found in bile)
to form micelles (Figure 23.34
2
).
Micelles
(mi-selz
9
) are col-
lections of fatty elements clustered together with bile salts in such
a way that the polar (hydrophilic) ends of the molecules face the
water and the nonpolar portions form the core. Also nestled in
the hydrophobic core are cholesterol molecules and fat-soluble
vitamins. Although micelles are similar to emulsion droplets,
they are much smaller “vehicles” and easily diffuse between mi-
crovilli to come into close contact with the apical cell surface.
Upon reaching the epithelial cells, the various lipid sub-
stances leave the micelles and move through the lipid phase of
the plasma membrane by simple diffusion (Figure 23.34
3
).
Without the micelles, the lipids will simply float on the surface of
the chyme (like oil on water), inaccessible to the absorptive sur-
faces of the epithelial cells. Generally, fat absorption is completed
in the ileum, but in the absence of bile (as might occur when a
gallstone blocks the cystic duct) it happens so slowly that most of
the fat passes into the large intestine and is lost in feces.
Once the free fatty acids and monoglycerides enter the epithe-
lial cells, the smooth ER converts them back into triglycerides.
Te triglycerides are then combined with lecithin and other phos-
pholipids and cholesterol, and coated with a “skin” of proteins
to form water-soluble lipoprotein droplets called
chylomicrons
(ki
0
lo-mi
9
kronz). Tese are dispatched to the Golgi apparatus
where they are processed for extrusion from the cell. Tis series
of events is quite different from the absorption of amino acids and
simple sugars, which pass through the epithelial cells unchanged.
Te milky-white chylomicrons are too large to pass through
either the plasma membrane of the epithelial cell or the base-
ment membrane of a blood capillary. Instead, the chylomicron-
containing vesicles migrate to the basolateral membrane and are
extruded by exocytosis (Figure 23.34
4
). Tey then enter the
more permeable lacteals. Tus, most fat enters the lymphatic
stream for distribution in the lymph. Eventually the chylomi-
crons are emptied into the venous blood in the neck region via
the thoracic duct, which drains the digestive viscera.
While in the bloodstream, the triglycerides of the chylomi-
crons are hydrolyzed to free fatty acids and glycerol by
lipopro-
tein lipase
, an enzyme associated with capillary endothelium.
Te fatty acids and glycerol can then pass through the capillary
walls to be used by tissue cells for energy or stored as fats in adi-
pose tissue. Liver cells then combine the residual chylomicron
material with proteins, and these “new” lipoproteins are used to
transport cholesterol in the blood.
Passage of short-chain fatty acids is quite different from what
we have just described. Tese fat breakdown products do not
depend on the presence of bile salts or micelles and are not re-
combined to form triglycerides within the intestinal cells. Tey
simply diffuse into the portal blood for distribution.
food materials (largely plant fibers such as cellulose), and millions
of bacteria. Tis debris is passed on to the large intestine.
±o understand absorption, remember that the fluid mosaic
structure of the plasma membrane dictates that nonpolar sub-
stances, which can dissolve in the lipid core of the membrane,
can be absorbed passively, and that polar substances must be
absorbed by carrier mechanisms. Most nutrients are absorbed
through the mucosa of the intestinal villi by
active transport
processes driven directly or indirectly (secondarily) by meta-
bolic energy (A±P). Tey then enter the capillary blood in the
villus to be transported in the hepatic portal vein to the liver.
Te exception is some lipid digestion products, which are ab-
sorbed passively by diffusion and then enter the lacteal in the
villus to be carried via lymphatic fluid to the blood.
Because tight junctions join the epithelial cells of the intestinal
mucosa at their apical surfaces, substances cannot move
between
cells for the most part. Consequently, materials must pass through
the epithelial cells and into the interstitial fluid abutting their baso-
lateral membranes to enter the blood capillaries.
As we describe the absorption of each nutrient class, you may
want to refer to the absorption summary on the right-hand side
of Figure 23.32 (p. 893).
Carbohydrate Absorption
Glucose and galactose, liberated by the breakdown of starch and
disaccharides, are shuttled by secondary active transport with
Na
1
into the epithelial cells
(Figure 23.35)
. Tey then move
out of these cells by facilitated diffusion and pass into the capil-
laries via intercellular cle²s. By contrast, fructose moves entirely
by facilitated diffusion (to enter and exit the cells).
Protein Absorption
Several types of carriers transport the different amino acids re-
sulting from protein digestion. Most of these carriers, like those
for glucose and galactose, are coupled to the active transport
of sodium (see Figure 23.33
2
). Short chains of two or three
amino acids (dipeptides and tripeptides, respectively) are ac-
tively absorbed by H
1
-dependent cotransport. Tey are di-
gested to their amino acids within the epithelial cells. Te amino
acids enter the capillary blood by diffusion (Figure 23.33
3
).
Homeostatic Imbalance
23.17
Whole
proteins are not usually absorbed, but in rare cases intact
proteins are taken up by endocytosis and released on the oppo-
site side of the epithelial cell by exocytosis. Tis process is most
common in newborn infants, reflecting the immaturity of their
intestinal mucosa (gastric acid secretion does not reach normal
levels until weeks a²er birth, and the mucosa is leakier than it
is later.) Absorption of whole proteins accounts for many early
food allergies: Te immune system “sees” the intact proteins as
antigenic and mounts an attack. Tese allergies usually disap-
pear as the mucosa matures.
Tis mechanism may also provide a route for IgA antibod-
ies in breast milk to reach an infant’s bloodstream. Tese anti-
bodies confer some passive immunity on the infant, providing
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